Ewolucja i tektonika staropaleozoicznego kompleksu strukturalnego południowego Bałtyku

Abstract

Przedstawiono charakterystykę tektoniczno-strukturalną utworów starszego paleozoiku południowego Bałtyku i przyległej strefy lądowej, wyróżniając 21 jednostek. Na podstawie analizy paleostrukturalnej zrekonstruowanej na 6 mapach odtworzono zmienność rozwoju subsydencji basenu staropaleozoicznego od wendu - kambru późnego po dewon górny. Określono wiek powstania syneklizy bałtyckiej i czas jej uformowania udowadniając, że jest ona, tak jak strefa T -T, strukturą heterochroniczną.

Evolution and tectonics of the Lower Palaeozoic structural complex in the southern Baltic

Structural foundations of the Baltic Syneclise should be connected with a pre-platform stage of evolution of the Southern slope of the Baltic Shield. Fragments of the oldest cover, dated as Jotnian (about 1.5 MA) are known among others from Gotska Sandon Island. In the majority of the area the sedimentary cover begun with the Vendian clastic deposits described as fluvatile fan deposits (K. Jaworowski, 1979, 1982) gradually passing into the Lower Cambrian deposits. According to K. Łydka et al. (1984) the Vendian series may continue higher on than it was assumed. The Precambrian sedimentary rocks are separated by well marked disconformity from the younger rocks of Caledonian tectonic cycle only locally in the eastern part of the syneclise. Configuration of the Vendian-Lower Cambrian sedimentary basin was reconstructed on the basis of simplified palaeostructural analysis (Fig. 2); the meridional hemianticlinal Oland Elevation, parallel Klaipeda Elevation and diagonal Kaliningrad Elevation were distinguished. The subsidence index for Vendian-Lower Cambrian is 0.91 to 4.1 mm/l000 years, for Lower Cambrian 3.3-15 mm/l000 years. In the Middle and Upper Cambrian took place a structural reconstruction and transformation of early palaeostructures (Fig. 3). The subsidence index in Middle Cambrian was 4.35 to 14.3 mm/1000 years, in Upper Cambrian merely 0.95 mm/l000 years. A next reconstruction of the basin took place in Ordovician (Fig. 4); palaeostructures of Bornholm and Słupsk were well marked. The subsidence axis was displaced north-east and in the Liepaja region an immersion exceeded 2000 m; the subsidence index is small and is equal from 0.75 to 3.7 mm/l000 years. These palaeostructures were transformed as a result of the Lower Silurian subsidence (Fig. 5), which rapidly increased near the marginal zone of the platform. The subsidence index ranges from 11.5 to 115 mm/l000 years. Deepending on the basin bottom continued in Upper Silurian and exceeded 250 mm/l000 years near the marginal zone. The Devonian subsidence was considerably lower, the system of labile zones (Fig. 6) differed from that of Silurian. In the area of the Polish part of the Baltic Syneclise, within the Caledonian structural complex the following structural stages were distinguished: Vendian - Middle Cambrian, CambrianTremadocian and Arenigian - Gedinnian. After W.J. Chajn (1974) in the Caledonian tectonic cycle there were distinguished in the syneclise the early stage represented by the red continental subformation and transgressive terrigenous formation (Vendian - Tremadocian), middle stage - carbonate clayey (Arenigian - Wenlock), late stage - regressive marine terrigenous-clayey and evaporite-carbonate formation.

The Baltic Syneclise is a heterogenous structure, differentiately shaped in time. The Masovian- Byelorussian Massif building the SE framework of the syneclise was uplifted as first (Lower – Middle Devonian), the north-western part as the latest (after Visean). The end of differentiated uplifting movements in the eastern part took place in Zechstein - Lower Triassic (F. Stolarczyk, 1980). Influence of the Alpine tectonic epoch expressed mainly by development of discontinuous tectonics was more dominant in the SW near-margin part of the platform than in the central and eastern area. Recently, the Baltic Shield and most part of the Baltic Syneclise influences isostatic uplifting movements, most intensive in the Bothnia Gulf (Fig. 8). The Lower Palaeozoic complex has been divided into 21 strongly dislocated structural units (Fig. 9). There have been distinguished a lot of lineaments that may determine deep fractures in the crystalline basement or in sedimentary cover. A part of lineaments overlaps with fault lines, determined by geophysical-geological survey (Fig. 9). NW - SE and NNE - SSW directions are dominant; the magnitude of the faults thrust in the western part is tremendous (over 3 km) but in the remaining area is multiply smaller - tens to one hundred meters. These faults were formed in various tectonic epochs, often they were regenerated over old structural foundations. The Variscan dislocation are dominant.